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F30 / F31 / F32 / F33 / F36 (2012 - current)
The sixth generation 3 series, chassis code F30. 2013 model year 328i and 335i sedans now in production. Read the F30 frequently asked question thread for all your basic question and dive into all the details in the ultimate F30 information thread.

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  #51  
Old 07-22-2012, 10:14 AM
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captainaudio captainaudio is offline
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Originally Posted by Elk View Post
This illustrates my point precisely. "xDrive ensures that none of drive power is wasted on a loss of traction: every kilowatt of power is effectively brought to bear on the road."

Note there is no claim of additional traction added which is not already present. Rather, "drive power" is not wasted.

AWD does not provide additional cornering traction. It only assists in avoiding wheel spin when accelerating. That is, no "drive power is wasted on a loss of traction."

Only if your accident avoidance maneuvers involve aggressive acceleration which would break the rear tires loose of a RWD vehicle does AWD provide you with a safety advantage.
Traction is a function of the tires grip on the road surface so a drive system can't provide more traction. What BMW does seem to be claiming is that X-Drive's dynamic distribution of power to the wheels increases stability.

AWD will not provide more braking power or cornering grip and I think that the erroneous belief that it does causes many drivers to be overconfident which gets them into troiuble,

CA
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Last edited by captainaudio; 07-22-2012 at 03:51 PM.
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  #52  
Old 07-22-2012, 12:03 PM
Elk Elk is offline
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Originally Posted by captainaudio View Post
Traction is a function of the tires grip on the road surface so a drive system can't provide more traction What BMW does seem to be claiming is that X-Drive's dynamic distribution of power to the wheels increases stability.

AWD will not provide more braking power or cornering grip and I think that the erroneous belief that it does causes many drivers to be overconfident which gets them into troiuble.
Agreed on all points, although Xdrive cannot provide greater stability in any circumstance other than when one is accelerating and traction is limited on a driven wheel. It can be beneficial powering out of a traction limited corner however, enhancing both acceleration and control.

Driver misapprehension of the system, coupled with resulting overconfidence, is what gets a lot of AWD owners in trouble.
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  #53  
Old 07-22-2012, 03:08 PM
IRBD IRBD is offline
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I love this lively exchange of ideas. I'm glad that traction and kW and drive efficiency have been addressed. So using the same dismissive arguments, can you tell me what the difference is between front wheel drive and rear wheel drive, assuming the front / rear weight bias was equal? From what you say they should be the same. Oh, so it might be of benefit to have driven wheels on all four corners? And of course the 150 pounds of additional weight make the AWD handle more poorly (unless your RWD has a passenger, a variable amount of stuff in the trunk, a full tank is, or your BMI is greater than 29). The best winter performance (including temperatures below 40 degrees) under real world conditions both starting and stopping comes in a package with AWD and winter tires. For warmer weather you put the summers back on.
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  #54  
Old 07-22-2012, 03:17 PM
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voip-ninja voip-ninja is offline
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Number of wheels being driven has no effect on stopping power, it's solely the brakes and rubber and road conditions. The added weight (about 200 lbs) might offer marginal improvement in stopping but I doubt it's much.
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  #55  
Old 07-22-2012, 03:37 PM
IRBD IRBD is offline
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I get that. That's why I said with winter tires. The driven wheels get you the 'go' and the tires get you the 'stop'.
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  #56  
Old 07-22-2012, 04:09 PM
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captainaudio captainaudio is offline
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Originally Posted by IRBD View Post
I love this lively exchange of ideas. I'm glad that traction and kW and drive efficiency have been addressed. So using the same dismissive arguments, can you tell me what the difference is between front wheel drive and rear wheel drive, assuming the front / rear weight bias was equal? From what you say they should be the same. Oh, so it might be of benefit to have driven wheels on all four corners? And of course the 150 pounds of additional weight make the AWD handle more poorly (unless your RWD has a passenger, a variable amount of stuff in the trunk, a full tank is, or your BMI is greater than 29). The best winter performance (including temperatures below 40 degrees) under real world conditions both starting and stopping comes in a package with AWD and winter tires. For warmer weather you put the summers back on.
FWD can be an advantage in smaller light cars like a mini where the majority of weight is in the front because the weight is over the driven wheels. There is also a certain advantage due to the fact that the wheels that are driving the car are also turning the car so in a sense the car is being "pulled" in the intended direction. With rear wheel drive where the rear wheels are "pushing" the car if the front wheels loose much of their grip you can wind up with extreme understeer. A good example of this would be when you attempt to make a turn on an icy surface and the car keeps on going straight. This will cause many inexperienced drivers to turn the steering wheel even more which will make the situation worse,

With light cars that had most of the weight in the back and were rear wheel drive (i.e. the orginal VW Beetle) due to the fact that most of the weight was over the (rear) drive wheels they tended to be decent winter cars.

Probably the worst situation is a RWD car that has the majority of the weight in the front.



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Last edited by captainaudio; 07-22-2012 at 04:12 PM.
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  #57  
Old 07-22-2012, 05:00 PM
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Originally Posted by voip-ninja View Post
The added weight (about 200 lbs) might offer marginal improvement in stopping but I doubt it's much.
More weight increases stopping distance. The increase in momentum more than offsets the increase in the coefficient of friction between tires and pavement.
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  #58  
Old 07-22-2012, 05:18 PM
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Originally Posted by captainaudio View Post
Probably the worst situation is a RWD car that has the majority of the weight in the front.
Indeed, such as a pick-up. They are a handful in the winter.

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With light cars that had most of the weight in the back and were rear wheel drive (i.e. the orginal VW Beetle) due to the fact that most of the weight was over the (rear) drive wheels they tended to be decent winter cars.
Yes, and fun if you understand their handling characteristics - although they can bite.

Many high performance cars and race cars have a rear weight bias. For example, F1 cars have roughly a 55% rear weight bias as it provides better handling. Of course, like everything in F1, this is subject to regulation. An SCCA Trans-AM car often has 60+% rear weight bias. At the other extreme, a NASCAR vehicle is often a couple percent forward bias as they are constantly turning and do relatively little braking. They also place 5%-10% on the left side as well. (Yes, I know BMW claims 50/50 front/rear is best. Period. It is great marketing.)

Nice diagram BTW, very helpful.
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  #59  
Old 07-22-2012, 10:30 PM
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The BMW AWD system does not provide additional control once the car is moving, especially not additional cornering traction. This explains why most vehicles in the ditch in winter driving are AWD; owners mistakenly believe they have better traction and can get away with more.
We owned a WRX for many years, it's definitely better in rain than 2wd, you can plow through puddles at highway speeds that would yank the wheel out of my hands on my 330i.

Or you can slow down a bit - never ever had any issues in the rain with the 330i, and with summer tires it rocks in the rain.

AWD does help cornering in snow, sand or liquid, since with the physics of those surfaces, a spinning tire has more traction than a rolling one. But you need to be on the gas through turns.
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  #60  
Old 07-22-2012, 10:36 PM
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Originally Posted by captainaudio View Post
Traction is a function of the tires grip on the road surface so a drive system can't provide more traction. What BMW does seem to be claiming is that X-Drive's dynamic distribution of power to the wheels increases stability.

AWD will not provide more braking power or cornering grip and I think that the erroneous belief that it does causes many drivers to be overconfident which gets them into troiuble,

CA
Only true on dry pavement - snowy pavement behaves differently, it's why WRC cars are always spinning their tires on snow, you get more traction that way, and 4 spinning tires are better than two. Which of course means you'll be going faster when you realize AWD doesn't help you stop.
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  #61  
Old 07-23-2012, 07:36 AM
OBS3SSION OBS3SSION is offline
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Only true on dry pavement - snowy pavement behaves differently, it's why WRC cars are always spinning their tires on snow, you get more traction that way, and 4 spinning tires are better than two. Which of course means you'll be going faster when you realize AWD doesn't help you stop.
WRC drivers use the car's power and AWD to "steer" instead of relying only on the turning of the steering wheel and grip of the tires on the asphalt/gravel/snow. You'll even see a similar style of driving with RWD cars using liberal oversteer and throttle to turn the car. But no road car driver is going to do that on public roads... or at least they should not be.

Oh, and WRC cars with their AWD don't run snow stages with all-season tires. They use some very aggressive studded winter tires.
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  #62  
Old 07-23-2012, 08:04 AM
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Originally Posted by OBS3SSION View Post
WRC drivers use the car's power and AWD to "steer" instead of relying only on the turning of the steering wheel and grip of the tires on the asphalt/gravel/snow. You'll even see a similar style of driving with RWD cars using liberal oversteer and throttle to turn the car. But no road car driver is going to do that on public roads... or at least they should not be.

Oh, and WRC cars with their AWD don't run snow stages with all-season tires. They use some very aggressive studded winter tires.
On RWD cars trail brake rotation is a technique to "rotate" the car and help get it pointed in the right direction. You have to be approaching the limits of grip to accomplish this and it is not something that should be done on public roads.

CA
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  #63  
Old 07-23-2012, 08:10 AM
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Originally Posted by OBS3SSION View Post
WRC drivers use the car's power and AWD to "steer" instead of relying only on the turning of the steering wheel and grip of the tires on the asphalt/gravel/snow. You'll even see a similar style of driving with RWD cars using liberal oversteer and throttle to turn the car. But no road car driver is going to do that on public roads... or at least they should not be.

Oh, and WRC cars with their AWD don't run snow stages with all-season tires. They use some very aggressive studded winter tires.
The reason WRC drivers do this has nothing to do with power or even AWD, it's because snow and other particulates (sand, gravel) behave like water, not a solid surface - the tire gets higher traction if the tire is spinning. That's the opposite of dry pavement, where a spinning tire loses traction.

So in this case, 4 driven wheels will corner faster than two, due to higher traction.

This is practical in real life at normal speeds in snow - you can corner much better in an AWD car if you stay on the gas in a corner.
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  #64  
Old 07-23-2012, 01:00 PM
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Last edited by captainaudio; 07-23-2012 at 01:02 PM.
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  #65  
Old 07-23-2012, 11:13 PM
skidaddy skidaddy is offline
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Recently got a new E90 xDrive, too good a deal to pass up, and kids are out of the house. Had a new 78 320i so you can guess my demographics. Find the conversation interesting as I have not been able to try the xDrive in the snow but so far have been pleased in the rain believe it helps with steering though can't compare it to RWD.

So my limited .02. I ski 40+ days and 10 of those with fresh snow. Live at 7,200 ft and ski area 7 miles away bases over at over 10,000. Lots of turns including switch backs with 7%+ inclines. My somewhat retired daily driver is a FZJ80 (Toyota Land cruiser). Numerous times I would track with a WRS type vehicle, let's say a recent WRX with Blizzek's.

Cruiser is a heavy vehicle (decent torque) with Revo AT tires (13/16 tread) could out corner the WRS snow "drifting" as they lose uphill momentum coming out of the turn, would both go in at same speed. Try this only on the mountains side turns. Was in a much better shape if went off the road as engaging the lockers was all it took to get back on the road. Would offer a tow though.

Last edited by skidaddy; 07-23-2012 at 11:20 PM.
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